Driving method for display apparatus

ABSTRACT

A display apparatus is capable of performing a gradation display and a high-speed low-power-consumption display by controlling the colored state of each pixel to an appropriate state. A driving method for the display apparatus displays images through deposition and dissolution of a metal by impressing a predetermined voltage on each pixel. A voltage pulse or AC voltage having an amplitude of not more than a threshold for deposition of the metal is impressed, and a writing pulse or an erasing pulse is controlled according to the resultant current. Alternatively, an additional writing pulse is impressed when the current is lowered to or below a predetermined value, after the writing. Further, the state of the pixel is detected through the current, and a rewriting operation is selected according to the detected state.

TECHNICAL FIELD

The present invention relates to a method of driving a display apparatusof the metal deposition type in which display is conducted throughdeposition and dissolution of a metal, and particularly to a method ofdriving a display apparatus which is suited to the so-called electronicpaper or the like.

BACKGROUND ART

In recent years, attendant on the spread of networks, documents hithertodistributed in the form of printed matter have come to be transmitted inthe form of the so-called electronic documents. Further, books andmagazines have come to be often provided in the form of the so-calledelectronic publishing. In order to read these pieces of information,reading from CRTs (cathode ray tubes) and liquid crystal displays ofcomputers has conventionally been widely conducted.

However, in a light emission type display such as the CRT, it has beenpointed out that the display causes conspicuous wearing on an ergonomicground and, therefore, is unsuited to long-time reading. In addition,even a light reception type display such as a liquid crystal display issaid to be similarly unsuited to reading, because of the flickeringwhich is intrinsic of fluorescent tubes. Furthermore, both of the typeshave the problem that the reading place is limited to the places wherecomputers are disposed.

In recent years, reflection type liquid crystal displays not using abacklight have put to practical use. However, the reflectance innon-display (display of white color) of the liquid crystal is 30 to 40%,which means a considerably bad visibility, as compared with thereflectance of printed matter printed on papers (the reflectance of OApapers and pocket books is 75%, and the reflectance of newspapers is52%). In addition, the glaring due to reflectors and the like are liableto cause wearing, which also is unsuited to long-time reading.

In order to solve these problems, the media so-called paper-likedisplays or electronic papers have been being developed. These mediamainly utilize coloration by moving colored particles between electrodesthrough electrophoresis or by rotating dichroic particles in an electricfield. In these methods, however, the gaps between the particles absorblight, with the result that contrast is worsened and that apractical-use writing speed (within 1 sec) cannot be obtained unless thedriving voltage is 100 V or higher.

As compared with the displays of these systems, electrochemical displaydevices for color development based on an electrochemical action(electrochromic display: ECD) are better in contrast, and they havealready been put to practical use as light control glass or timepiecedisplays. It should be noted here that the light control glass andtimepiece displays are not directly suited to the electronic paper orthe like uses, since it is intrinsically unnecessary to perform matrixdriving. Besides, they are generally poor in quality of black color, andthe reflectance thereof remains at a low level.

In addition, in such displays as electronic papers, they are continuedlyexposed to solar light or room light on a use basis. In anelectrochemical display device put to practical use in the light controlglass and timepiece displays, an organic material is used for formingblack-colored portions. This leads to a problem concerning lightresistance. In general, organic materials are poor in light resistance,and the black color density thereof is lowered through fading when usedfor a long time.

In order to solve these technical problems, there has been proposed anelectrochemical display device using metal ions as a material for colorchange. In the electrochemical display device, the metal ions arepreliminarily mixed into a polymer electrolyte layer, the metal isdeposited and dissolved by electrochemical redox reactions, and thechange in color attendant on this is utilized to perform display. Here,for example, when the polymer electrolyte layer contains a coloringmaterial, it is possible to enhance the contrast in the case where thecolor change occurs.

Meanwhile, in the metal deposition type electrochemical display devicebased on deposition and dissolution of the metal, a threshold voltagewhich is the deposition overvoltage is utilized to achieve display. Ineach pixel, the metal is deposited when a minus voltage in excess of thethreshold voltage is impressed between electrodes, whereas the metal isdissolved when a plus voltage is impressed between the electrodes.

In such a metal deposition type electrochemical display device, it isdifficult to control the colored state, which is a great problem inenhancing the quality of display. For example, according to theconventional thought, the metal deposition type display device isbasically designed for black-and-white display, and few attempts havebeen made to achieve display of gradation. This is due to the fact thatit is difficult to control the colored state to an intermediate coloredstate with good reproducibility.

Besides, in the metal deposition type electrochemical display device, ittakes a certain period of time to deposit or dissolve the metal and,therefore, problems are left as to response speed and power consumption.For example, when it is tried to once erase all pixels and to performnew writing at the time of rewriting the screen, it takes a considerablelength of time and the power consumption is great.

Furthermore, in the metal deposition type electrochemical displaydevice, there is the tendency that the display contrast would be loweredwith the lapse of time, and a countermeasure against this problem isdesired.

The present invention has been proposed in consideration of theabove-mentioned circumstances of the related art. Accordingly, it is anobject of the present invention to provide a method of driving a displayapparatus by which it is possible to control the colored state of eachpixel to an appropriate state, for example, a uniform state in a metaldeposition type display apparatus. It is another object of the presentinvention to provide a method of driving a display apparatus by which itis possible to achieve display of gradations and display with highdefinition. It is a further object of the present invention to provide amethod of driving a display apparatus by which it is possible tocontrive a higher display speed and a lower power consumption of theapparatus. It is yet another object of the present invention to providea driving method by which it is possible to sustain images for a longtime in a metal deposition type display apparatus.

DISCLOSURE OF INVENTION

In order to attain the above objects, the method of driving a displayapparatus according to the present invention is firstly characterized inthat the driving method is a method of driving a display apparatus fordisplaying an image through deposition and dissolution of a metal byimpressing a predetermined voltage on each pixel, wherein a voltagepulse or AC voltage having an amplitude of not more than a threshold fordeposition of the metal is impressed, and, according to the resultingcurrent, a writing pulse or an erasing pulse is controlled.

A metal deposition type display apparatus has a hysteresis incurrent-voltage characteristic such that when a negative impressedvoltage exceeds a threshold, a current begins to flow to causedeposition of a metal, and, thereafter, the current flowing condition ismaintained even if the voltage is reduced to or below the threshold. Inthe display apparatus having such a current-voltage characteristic, whenvoltage scanning is conducted by limiting the negative voltage range tothe range of not more than the threshold, the resistance in the vicinityof zero voltage is gradually raised as the colored density is lowered.

Where a pulse voltage or AC voltage having a voltage of less than thethreshold is impressed and the resulting current (namely, theresistance) is measured by utilizing the above-mentioned phenomenon, itis possible to detect the colored state. In the present invention, thenegative writing voltage or the positive erasing voltage which isimpressed is controlled according to the colored state detected, wherebydensity control is performed. By this operation, the colored density ofeach pixel is accurately controlled to a predetermined level.

The method of driving a display apparatus according to the presentinvention is secondly characterized in that the driving method is amethod of driving a display apparatus for forming an image throughdeposition and dissolution of a metal by impressing a predeterminedvoltage on each pixel, wherein, after writing is conducted by impressinga voltage of not less than a threshold for deposition of the metal, avoltage pulse or AC voltage having an amplitude of not more than athreshold for deposition of the metal is impressed, and, when theresulting current is decreased to or below a predetermined value, anadditional writing pulse is impressed.

As has been described above, the colored state can be grasped bymeasuring the above-mentioned current value, and the colored state (thedensity of the pixel) is maintained when additional writing by theadditional writing pulse is conducted when the current value isdecreased attendant on the progress of the dissolution of the metal.

The method of driving a display apparatus according to the presentinvention is thirdly characterized in that the driving method is amethod of driving a display apparatus for displaying an image throughdeposition and dissolution of a metal by impressing a predeterminedvoltage on each pixel, wherein a voltage pulse or AC voltage having anamplitude of not more than a threshold for deposition of the metal isimpressed, the writing condition of each pixel is detected according tothe resulting current, and a rewriting operation is selected accordingto the detection results.

In the case of the third-named invention, first, by measuring theabove-mentioned current value, it is detected whether the pixel state isthe colored state (black) or the non-colored state (white) on a pixelbasis. Then, according to the detection results and the black/whitestate of the pixels after rewriting, a selection is made among nooperation, erasure, writing, density additional writing, etc. Forexample, where a pixel in the black state is in the black state evenafter rewriting or where a pixel in the white state is in the whitestate even after rewriting, no operation is conducted for the relevantpixel. Where a pixel in the black state should be in the white stateafter rewriting or where a pixel in the white state should be in theblack state after rewriting, an erasing operation or a writing operationis conducted for the relevant pixel. By this method, it suffices toperform the driving a minimum required number of times, whereby therewriting can be achieved in a minimum time and with a minimum powerconsumption.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a general sectional view showing one example of a metaldeposition type display apparatus of a simple matrix system;

FIG. 2 is a general exploded perspective view showing one example of themetal deposition type display apparatus of the simple matrix system;

FIG. 3 is a general exploded perspective view showing one example of ametal deposition type display apparatus of an active matrix system;

FIG. 4 is a circuit diagram showing one example of the circuitconfiguration of the metal deposition type display apparatus of theactive matrix system;

FIG. 5 is a waveform diagram showing a triangular wave voltage impressedfor examining current-voltage transient response characteristic;

FIG. 6 is a characteristic diagram showing the current-voltage transientresponse characteristic;

FIG. 7 is a schematic view of a panel of 3×3 pixels;

FIG. 8 is a waveform diagram showing a driving voltage waveform in abasic driving method for a metal deposition type display apparatus;

FIG. 9 is a characteristic diagram showing the current-voltagecharacteristic of a display apparatus manufactured;

FIG. 10 is a characteristic diagram showing the difference incurrent-voltage characteristic when a writing scan voltage is varied;

FIG. 11 is a characteristic diagram showing the relationship betweencolored density (optical density) and complex impedance;

FIG. 12 is a waveform diagram showing one example of coloration control;

FIG. 13 is a waveform diagram showing one example of erasure control;

FIG. 14 is a waveform diagram showing one example of fading control;

FIG. 15 is a flowchart of one example in which rewriting of a screen iscontrolled by monitoring the condition before writing; and

FIG. 16 is a flowchart of one example in which gradation control isperformed by monitoring the condition before writing.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the method of driving a display apparatus to which the presentinvention has been applied will be described in detail below referringto the drawings.

Prior to the description of the driving method, first, an example ofconfiguration of the display apparatus which is the object of thepresent invention will be described.

The display apparatus in this example is an electrochemical displayapparatus for displaying images through deposition and dissolution of ametal by utilizing electric deposition characteristics of the metal, andis driven by a simple matrix driving system. Therefore, the drivingelectrodes consist of a first electrode group X₁, X₂ . . . and a secondelectrode group Y₁, Y₂ . . . orthogonal thereto, which areintersectingly arranged in a lattice pattern. FIGS. 1 and 2 show aspecific structure of the driving electrodes, in which stripe formtransparent column electrodes 2 corresponding to the first electrodegroup are formed on a transparent substrate 1. A base substrate 3provided thereon with stripe form counter electrodes (row electrodes) 4corresponding to the second electrode group is disposed opposite to thetransparent substrate 1, and these substrates are laid on each other,with a polymer electrolyte layer 5 therebetween. The transparent columnelectrodes 2 and the row electrodes 4 are provided in predeterminednumbers according to the desired number of pixels, and the intersectionsof the two kinds of electrodes constitute pixels. With a voltageimpressed selectively between the transparent column electrodes 2 andthe row electrodes 4, a metal 6 is deposited, with the result ofcoloring of the pixels.

In the above configuration, a transparent glass substrate such as aquartz glass plate and a white plate glass plate can be used as thetransparent substrate 1, but the transparent glass substrate is notlimitative. Examples of the material which can be used for thetransparent substrate 1 include esters such as polyethylene naphthalate,polyethylene terephthalate, etc., polyamides, polycarbonates, celluloseesters such as cellulose acetate, etc., fluoro polymers such aspolyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylenecopolymer, etc., polyethers such as polyoxymethylene, etc., polyacetal,polystyrene, polyolefins such as polyethylene, polypropylene,methylpentene polymer, etc., and polyimides such as polyimide-amides,polyether imides, etc. Where one of these synthetic resins is used as asupport, a rigid substrate which cannot easily be bent can be obtained,and a flexible film form substrate can also be obtained.

For the transparent electrodes 2, it is preferable to use, for example,a mixture of In₂O₃ and SnO₂, i.e., the so-called ITO film or a filmcoated with SnO₂ or In₂O₃. Films obtained by doping the ITO film or theSnO₂- or In₂O₃-coated film with Sn or Sb can also be used, and, further,MgO, ZnO and the like can also be used.

On the other hand, examples of the matrix polymer to be used for thepolymer electrolyte layer 5 include polyethylene oxide,polyethyleneimine, and polyethylene sulfide, the skeleton units of whichare represented by the formulas —(C—C—O)_(n)—, —(C—C—N)_(n)—, and—(C—C—S)_(n)—. With the skeleton units as main chain structures,branching may be present. Besides, polymethyl methacrylate,polyvinylidene fluoride, polyvinylidene chloride, polycarbonates and thelike are also preferable for this use.

In forming the polymer electrolyte layer 5, a required plasticizer ispreferably added to the matrix polymer. Preferred examples of theplasticizer include water, ethyl alcohol, isopropyl alcohol, andmixtures thereof in the case where the matrix polymer is hydrophilic,and include propylene carbonate, dimethyl carbonate, ethylene carbonate,γ-butyrolactone, acetonitrile, sulfolane, dimethoxyethane, ethylalcohol, isopropyl alcohol, dimethyl formamide, dimethyl sulfoxide,dimethyl acetamide, n-methylpyrrolidone, and mixtures thereof in thecase where the matrix polymer is hydrophobic.

The polymer electrolyte layer 5 is formed by dissolving an electrolytein the matrix polymer. Examples of the electrolyte includes not onlymetallic salts capable of functioning as a color forming material fordisplay but also quaternary ammonium halides (F, Cl, Br, I), alkalimetal halides (LiCl, LiBr, LiI, NaCl, NaBr, NaI, etc.), alkali metalcyanides, and alkali metal thiocyanides, and a material containing atleast one support electrolyte selected from these compounds is dissolvedas an electrolyte. Here, examples of the metal ions constituting themetallic salt capable of functioning as a color forming material fordisplay include bismuth, copper, silver, lithium, iron, chromium,nickel, and cadmium, which are used either singly or in combination. Themetallic salt may be an arbitrary salt of any of these metals; examplesof the suitable silver salt include silver nitrate, silver borofluoride,silver halide, silver perchlorate, silver cyanide, and silverthiocyanide.

Besides, a coloring material may be added to the polymer electrolytelayer 5, for enhancing contrast. Where the coloration by deposition ofthe metal occurs in black, it is preferable to use white color as thebackground color, and a white material having a high hiding property ispreferably introduced as a coloring material. As such a material, whiteparticles for coloration can be used. Examples of the white particlesfor coloration include titanium dioxide, calcium carbonate, silica,magnesium oxide, and aluminum oxide.

In the case of inorganic particles, the ratio in which the white pigmentis mixed is preferably about 1 to 20% by weight, more preferably about 1to 10% by weight, and further preferably about 5 to 10% by weight. Thereason for restricting the mixing ratio in this manner is as follows.The white pigments such as titanium oxide is not soluble in polymers butare merely dispersed in the polymers; therefore, when the mixing ratiois increased, the white pigment is coagulated, resulting innonuniformity of optical density. Besides, the white pigments lack ionicconductivity, so that an increase in the mixing ratio leads to alowering in the conductivity of the polymer electrolyte. Inconsideration of both of these points, the upper limit of the mixingratio is about 20% by weight.

In the case where the inorganic particles are mixed into the polymerelectrolyte layer 5 as a coloring material as above-mentioned, thethickness of the polymer electrolyte layer 5 is preferably in the rangeof 10 to 200 μm, more preferably 10 to 100 μm, and further preferably 10to 50 μm. As the polymer electrolyte layer 5 is thinner, the resistancebetween the electrodes is lower, which favorably leads to reductions incolor forming and decoloring times and to a reduction in powerconsumption. However, when the thickness of the polymer electrolytelayer 5 is reduced to below 10 μm, mechanical strength is lowered, andinconveniences such as generation of pinholes and cracks would occur.Besides, when the thickness of the polymer electrolyte layer 5 is toosmall, the mixing ratio of the inorganic particles is low accordingly,and the whiteness property (optical density) may be insufficient.

Incidentally, where a coloring matter is used as the coloring materialmixed in the polymer electrolyte layer 5, the mixing ratio of thecoloring material may be not more than 10% by weight. This is becausethe color forming efficiency of a coloring matter is by far higher thanthat of inorganic particles. Therefore, when a coloring matter which iselectrochemically stable is used, a sufficient contrast can be obtainedeven if the coloring matter is used in a small amount. As such acoloring matter, for example, oil-soluble dyes are preferred.

The base substrate 3 provided on the back side may not necessarily betransparent, and substrates, films and the like capable of securelyholding the row electrodes 4 can be used as the base substrate 3.Examples of the usable substrates, films and the like include glasssubstrates such as quartz glass plates, white sheet glass plate, etc.,ceramic substrates, paper substrates, and wood substrates. Thesenaturally are not limitative, and synthetic resin substrates and thelike can also be used. Examples of the materials of the synthetic resinsubstrates include esters such as polyethylene naphthalate, polyethyleneterephthalate, etc., polyamides, polycarbonates, cellulose esters suchas cellulose acetate, etc., fluoro polymers such as polyvinylidenefluoride, polytetrafluoroethylene-cohexafluoropropylene, etc.,polyethers such as polyoxymethylene, etc., polyacetal, polystyrene,polyolefins such as polyethylene, polypropylene, methylpentene polymer,etc., and polyimides such as polyimide-amides, polyether amides, etc.Where any of these synthetic resins is used for the base substrate, arigid substrate which would not easily be bent can be obtained, and aflexible film form substrate can also be obtained.

For the row electrodes 4, conductive materials, for example, metallicmaterials can be used. It should be noted here, however, that when thereis a large potential difference between the metal constituting the rowelectrodes 4 and the metal to be deposited on the transparent columnelectrodes 2, there is the possibility that electric charges may beaccumulated on the electrodes in the colored condition, and migration ofthe electric charges may occur, to cause coloration of unintendedpixels. Particularly when the potential difference exceeds thedeposition overvoltage at the time of deposition of the metal (thethreshold in simple matrix driving), the above-mentioned coloration maytake place. In view of this, a metal such that the potential differencebetween itself and the metal to be deposited as the color formingmaterial is less than the deposition overvoltage (threshold) isdesirably selected as the material of the row electrodes 4. Ideally, thematerial in the unionized state (metallic state) of the metal ions usedas the color forming material is used as the metallic materialconstituting the row electrodes 4. Namely, the same metal as the metalto be deposited and dissolved is used for the row electrodes 4; forexample, in the case of utilizing deposition and dissolution of silver,silver is used for the row electrodes 4. This precludes the possibilitythat the above-mentioned potential difference might be generated in thecondition where the metal is deposited on the transparent columnelectrodes 2.

Other materials which can be used to constitute the row electrodesinclude platinum (Pt), gold (Au), and carbon (C). The use of the samemetal, for example silver (Ag), as the species of the metal ions to beused for the deposition-dissolution reaction as the material of the rowelectrodes (counter electrodes) as above-mentioned is the most idealsystem, in view of the mechanism of the reaction and the like. However,in the case where silver of the row electrodes is formed only in athickness on the order of several hundreds of nanometers on a processbasis in enhancing the fineness of the simple matrix device, therepetition of the deposition-dissolution reaction causes superpositionof irreversible reactions such as dendrite growth, resulting in thatsilver of the row electrodes is gradually consumed and, eventually, theelectrode itself may be lost. In the condition where the row electrodehas been dissolved and lost, an extremely bad influence is exerted notonly on the pixel corresponding to the lost electrode but also on thedisplay as a whole, in view of the principle of the simple matrixdriving. In consideration of this problem, platinum and gold can be usedfor the electrodes, as a metal which is nobler (stabler) on a potentialbasis than silver subjected to the deposition-dissolution reaction.Incidentally, the metals nobler than silver are only these two metals(platinum and gold); where one of the metals which are baser than silveron a potential basis (the metals other than silver, platinum, and gold)is used, the metal is unconditionally dissolved, and silver ionscontained in the liquid electrolyte composition would be deposited onthe surface of the metal in the form of replacement plating.

An additional description will be made of the phenomenon in which theabove-mentioned potential difference approaches zero in the conditionwhere the metal has been deposited on the transparent column electrodes2 as described above. Silver, platinum, and gold have the followingstandard electrode potentials (in aqueous solution):

Ag⁺ +e ⁻

Ag E₀=0.80 V,

Pt²⁺+2e ⁻

Pt E₀=1.18 V,

Au⁺ +e ⁻

Au E₀=1.83 V.

All the potential values are against the standard hydrogen electrodepotential (SHE), and platinum and gold will generate the potentialdifferences of Δ=0.38 V and 1.03 V, respectively. The generation of thepotential difference cannot be obviated in a perfect initial condition.However, as the deposition-dissolution reaction is repeated, Ag isfavorably left in an incompletely dissolved state on the platinum orgold electrodes, with the result that the potential difference Δapproaches zero.

While an example of configuration of the metal deposition type displayapparatus of the simple matrix driving system has been described above,screen rewriting at a higher speed can be realized by adopting an activematrix system in which each pixel is respectively driven by a circuitcomposed of one or a plurality of thin film transistors. FIG. 3 shows anexample of configuration of a display apparatus of the active matrixsystem.

Also in the display apparatus of the active matrix system, a transparentsubstrate 11 and a base substrate 12 are disposed to sandwichtherebetween an electrolyte layer 13 composed of a polymer electrolyteor the like, in the same manner as in the display apparatus of thesimple matrix system. Besides, the materials constituting thetransparent substrate 11, the base substrate 12, and the electrolytelayer 13 and the like are also the same as in the display apparatus ofthe simple matrix system. The display apparatus of the active matrixsystem differs largely from the display apparatus of the simple matrixsystem in electrode structure. First, the transparent substrate 11 isprovided thereon with a transparent electrode 14, which is formed on thewhole surface of the transparent substrate 11, instead of being formedin a stripe pattern, and serves as a common electrode. On the otherhand, dot form pixel electrodes 15 are arranged on the base substrate 12in a matrix form correspondingly to pixels, and a thin film transistorfor driving is connected to each of the pixel electrodes 15.

FIG. 4 shows the circuit configuration in the display apparatus of theactive matrix system. As has been described above, in the displayapparatus of the active matrix system, the pixels 24 each composed ofthe thin film transistor 21, the pixel electrode 22, and the commonelectrode 23 are arranged in a matrix form. In addition, data linedriving circuits 25 a, 25 b and a gate line driving circuit 26 forselectively driving each pixel 24 are provided. The data line drivingcircuits 25 a, 25 b and the gate line driving circuit 26 select apredetermined data line 28 or gate line 29 according to a control signalcoming from a signal control unit 27, and switch the ON/OFF condition ofeach pixel 24.

Next, a basic driving method for this display apparatus will bedescribed. In a display apparatus utilizing electric depositioncharacteristics, for example, in a display apparatus of the simplematrix system, when a triangular wave voltage as shown in FIG. 5 isimpressed between a column electrode and a row electrode, acurrent-voltage transient response characteristic as shown in FIG. 6 isdisplayed. Incidentally, FIG. 6 shows an example of characteristic inthe case where an Ag electrode is used as the row electrode and silverions and iodide ions are dissolved in the polymer electrolyte.

Referring to FIG. 6, as a voltage in the range from zero to the minusside is impressed between the column electrode and the row electrode,non-deposition of silver continues for a while, and deposition of silveronto the column electrode begins when the voltage exceeds a thresholdvoltage V_(th). This is seen from FIG. 6, in which a current attendanton deposition begins to flow when the voltage exceeds the thresholdvoltage V_(th). Thus, each pixel has such a characteristic that acharacteristic as capacitor predominates before deposition (white) andthat the resistance is lowered as the deposition (black) proceeds.

When the voltage once exceeds the threshold voltage V_(th) due to awriting voltage V_(w) corresponding to an apex of the triangular wavevoltage, the deposition of silver continues even when the voltage isgradually lowered, and the silver deposition continues even when thevoltage is less than the threshold voltage V_(th). The deposition ofsilver ends when the impressed voltage is lowered to a holding voltageV_(ke). In other words, once the impressed voltage exceeds the thresholdvoltage V_(th) and nuclei (seeds) are formed, the silver depositionoccurs even at a voltage of not more than the threshold voltage V_(th).

On the other hand, when a voltage in the reverse polarity (plus) isimpressed between the column electrode and the row electrode,dissolution of silver begins, and the deposited silver disappears at thetime when the impressed voltage reaches an erasing voltage V_(ith). Whena voltage of not less than this value is impressed, iodine is libratedto be deposited on the electrode, resulting in coloration in yellow.

Considering the driving of the display apparatus showing theabove-mentioned current-voltage transient response characteristic, thesimplest mode resides in that a voltage in excess of the above-mentionedthreshold voltage is impressed at the time of address driving, todeposit silver, thereby performing writing of the pixel. Hereinafter,for simplification of description, monochromic display by use of 3×3pixels shown in FIG. 7 will be taken as an example, and a basic drivingmethod for the metal deposition type display apparatus will bedescribed.

FIG. 8 shows one example of a driving voltage waveform for performingdisplay by utilizing the threshold voltage for deposition, in the metaldeposition type display apparatus such that silver is deposited on thecolumn electrode when a minus voltage impressed exceeds the thresholdvoltage V_(th) and that silver is dissolved when a plus voltage isimpressed. In FIG. 8, there are shown column voltages impressed oncolumn electrodes C₁, C₂, and C₃, row voltages impressed on rowelectrodes R₁, R₂, and R₃, and voltages impressed on pixels (C₁, R₁),(C₁, R₂), and (C₂, R₂).

At the time of display, a signal writing voltage pulse V_(sig) less thanthe threshold voltage V_(th) is impressed on each of the columnelectrodes C₁, C₂, and C₃, and a selecting voltage pulse V_(sel) lessthan the threshold voltage V_(th) is impressed on each of the rowelectrodes R₁, R₂, and R₃, for operations of sequential selection fromthe upper side. In this case, a voltage (writing voltage V_(w))(=V_(sig)−V_(sel)) in excess of the threshold voltage V_(th) isimpressed only on the pixels where silver is to be deposited, to effectdeposition of silver on the transparent column electrodes, wherebywriting is performed.

For example, at the pixels (C₁, R₁) and (C₂, R₂), the signal writingvoltage pulse V_(sig) on the column electrodes C₁ and C₂ and theselecting voltage pulse V_(sel) on the row electrodes R₁ and R₂ overlapwith each other, whereby the writing voltage V_(w) due to the voltagedifference between the voltage pulses is impressed, to effect silverdeposition (writing). On the other hand, at the pixel (C₁, R₂), thesignal writing voltage pulse V_(sig) on the column electrode C₁ and theselecting voltage pulse V_(sel) on the row electrode R₂ do not overlapwith each other in any period, and only one of the signal writingvoltage pulse V_(sig) and the selecting voltage pulse V_(sel), which areless than the threshold voltage V_(th), is impressed; therefore, silverdeposition does not take place, and writing on the pixel is notperformed.

After the writing, the display can be memorized by opening orshortcircuiting both the column electrodes and the row electrodes. Inaddition, when an erasing voltage −V_(e) is impressed on the rowelectrodes R₁, R₂, and R₃ at a predetermined timing so that a plusvoltage V_(e) is impressed on each pixel, dissolution of silver iseffected, whereby erasure is performed. Incidentally, the erasingvoltage V_(e) is set to be equal to or slightly lower than the erasingvoltage V_(ith) in FIG. 6 above. If the erasing voltage V_(e) exceedsthe erasing voltage V_(ith), coloration may occur.

While the basic driving method has been described above, in the presentinvention, in the display apparatus having the configuration as abovedescribed, a display in an appropriate density is realized and a displayof gradations can be conducted by a control method as described below.

When voltage scanning by limiting a negative voltage range is applied tothe current-voltage characteristic shown in FIG. 6, the resistance inthe vicinity of zero voltage is higher in the case of a non-coloredcondition or an insufficiently colored condition, as compared with thecase of complete coloration by scanning at a sufficient voltage. Theresistance is highest in the non-colored condition, and is lowest in thecase of complete coloration; the resistance has a value according to thedegree of coloration.

By utilizing this phenomenon and by first impressing a pulse voltage (orAC voltage) at a voltage less than the coloration threshold, measuringthe resulting current, i.e., the resistance (detection of condition) andsubsequently applying an appropriate amount of a negative writingvoltage, it is possible to control the density. For example, a voltagepulse of 0.1 volt and 10 ms is impressed as a detection pulse, and theresistance (current value) is measured. Next, a negative voltage pulseof 1 volt and 10 ms is impressed as a writing pulse. This cycle isrepeated until the resistance is brought to a target value, wherebycoloration can be achieved while constantly controlling the respectivedensities.

By carrying out such a coloration control, it is also possible toachieve a gradation display. When voltage scanning is conducted bylimiting the negative voltage range, the resistance in the vicinity ofzero voltage shows a value according to the degree of coloration.Therefore, it suffices to set each gradation and the relevant resistancevalue (current value) in correspondence with each other, and to performthe density control based thereon.

In the case of erasing a colored pixel, a positive voltage is impressedto cause an appropriate current to flow, whereby silver is dissolved,resulting in that the pixel is made transparent. As shown in FIG. 6,when a positive voltage is impressed in excess, not only the color iserased but also iodine is librated, to cause coloration in yellow, inthe case where silver iodide is used as dissolved ions, for example. Forachieving appropriate coloration while preventing this phenomenon, it iseffective to detect the resistance value and to control the erasure, inthe same manner as in the control of coloration mentioned above.

Specifically, when a cycle of first measuring the current, i.e., theresistance (detection of condition) by impressing a pulse voltage (or ACvoltage) at a voltage less than the coloration threshold andsubsequently applying a positive erasing voltage with an appropriateamplitude and an appropriate time is repeated, it is possible to achieveappropriate erasure and, by utilizing this, to control the density. Forexample, a positive or negative voltage pulse of 0.1 volt and 10 ms isimpressed as a detection pulse, and the resistance (current) ismeasured. Subsequently, a positive voltage pulse of 1 volt and 10 ms isimpressed as an erasing pulse. With this cycle repeated until theresistance is brought to a target value, it is possible to constantlycontrol the respective densities.

The above-mentioned phenomenon can be utilized also for control offading. In the above-mentioned display apparatus, by returning thevoltage to zero or opening the circuit after the coloration, the coloredstate can be maintained for a certain length of time (several tens ofminutes at room temperature); however, silver is gradually dissolved andthe coloration density (optical density) is gradually lowered, with thelapse of time. In order to maintain the density for a long time,appropriate writing according to the deficient density is conducted,which is effective also for minimizing the power consumption.

By applying the above-mentioned control, the current flowing due to thestate detecting pulse is detected, and an additional writing pulse isimpressed when the current is decreased to or below a predeterminedvalue, whereby the density can be maintained for a long time. As hasbeen described above, when voltage scanning is conducted by limiting anegative voltage range, the resistance in the vicinity of zero voltagerises as the density is lowered. In view of this, the state detectingpulse is periodically impressed after writing, the resultant current ismonitored, and, when the measured resistance exceeds a predeterminedlevel, it is judged that the density has been lowered, and an additionalwriting pulse is impressed.

Furthermore, an application to control of rewriting can be made. Forexample, in rewriting a black-white two-valued screen for displayingcharacters or the like, it can be contemplated to rewrite a currentscreen into a next screen according to the current screen condition bythe following technique. First, whether the current state of each pixelis white (non-colored state) or black (colored state) is detected by useof a state detecting pulse or a minute AC voltage. Next, rewriting isconducted through selection among “no operation”, “erasure”, “writing”,and “density adding writing”, according to the black/white state of thepixel after rewriting.

For example, where a pixel in the black state is to be in the blackstate also after rewriting or where a pixel in the white state is to bein the white state also after rewriting, no operation is conducted as arewriting operation. Where a pixel in the black state is to be in thewhite state after rewriting or where a pixel in the white state is to bein the black state after rewriting, an erasing operation or a writingoperation is conducted. This ensures that a minimum required operationis only needed, and the rewriting of a screen can be achieved in aminimum time and with a minimum power consumption.

EMBODIMENTS

Next, specific embodiments to which the present invention has beenapplied will be described, based on experimental results.

[Production of Stripe Form Electrodes]

First Electrodes (Display Electrodes): On a polycarbonate substratemeasuring 10 cm×10 cm and 0.2 mm in thickness, stripe form transparentelectrodes and an insulation film were formed in the followingprocedure. Incidentally, the stripe form transparent electrodes have astripe width of 150 μm and a pitch of 170 μm, and the size of an openingportion (a portion not covered with the insulation film) is 140 μmsquare.

First, an ITO film with a film thickness of 500 nm and a sheetresistance of 12Ω/□ was formed on the above-mentioned polycarbonatesubstrate by sputtering. Next, a photoresist was applied to the ITOfilm, and the photoresist was processed into a desired stripe shape bylithography. Followingly, the polycarbonate substrate was immersed in anITO etching solution, to remove the portions of the ITO film which werenot covered with the photoresist. Subsequently, the remainingphotoresist was removed by use of an organic solvent such as acetone.

Next, a film of SiO₂ was formed in a thickness of 200 nm on the stripeform ITO film by a plasma CVD method using TEOS (Tetra-EthoxyOrtho-Silicate: Si(OC₂H₅)₄) and O₂. A photoresist is applied to the SiO₂film, and the photoresist was processed into a desired stripe shape bylithography. The substrate was then immersed in a mixed solution ofammonium fluoride, hydrofluoric acid and the like, to remove theportions of the SiO₂ film which were not covered with the photoresist.Subsequently, the remaining photoresist was removed by use of an organicsolvent such as acetone.

Second Electrodes (Counter Electrodes): The counter electrodes wereproduced by the following procedure. Copper electrodes were formed in astripe form on a base substrate made of polycarbonate or PET(polyethylene terephthalate) or an epoxy-based resin such as aglass-epoxy resin used for circuit substrates. The stripe form copperelectrodes were formed by the method as follows. First, a film of copperwas formed on the entire surface of the base substrate by sputtering orthe like. Subsequently, a photoresist was applied to the whole surfaceof the copper electrode, the photoresist was covered with a metal maskor UV-shielding mask formed in a stripe pattern, followed by irradiationwith UV rays. The photoresist and the electrode in the areas masked bylithography were removed by wet etching or dry etching so that each ofthe stripe form electrodes was insulated. Next, an electric field wasapplied to the stripe form electrodes, which were immersed in a solutioncontaining silver or a desired metal dissolved therein, and silver wasdeposited on the electrodes by an electroplating method, to produce thecounter electrodes.

In the counter electrodes, the thickness of copper is about 15 μm, thethickness of silver is about 15 μm, and the total electrode thickness isabout 30 μm. Incidentally, an electroless plating method of depositingsilver or the metal without applying an electric field is also known,but the electroplating method is desirable because the thickness of themetal deposited is large.

The base substrate is desirably so formed that the interval of thepixels for display is as small as possible. Besides, in the basesubstrate, the gaps between the pixels are covered with a solidelectrolyte; on the assumption that the solid electrolyte is transparentor translucent, it is preferable to use a white substrate.

[Production of Display Electrodes, Preparation of Polymer SolidElectrolyte, and Application Thereof]

A mixture of 1 part by weight of polyvinylidene fluoride having amolecular weight of about 350,000, 10 parts by weight of dimethylsulfoxide, 1.7 parts by weight of silver iodide, and 1 part by weight ofsodium iodide was heated to 120° C., to prepare a uniform solution.Next, 0.2 part of titanium dioxide having an average particle diameterof 0.5 μm was added to the solution, and was dispersed uniformly by ahomogenizer. The resulting liquid was applied to the above-mentionedglass substrate in a thickness of 20 μm by use of a doctor blade, thenthe second electrodes were immediately adhered thereto, and theresulting assembly was vacuum dried at 110° C. under a pressure of 0.1MPa for 1 hr, whereby a gelled polymer solid electrolyte was formedbetween the two kinds of electrodes. Next, end faces of the laminatewere sealed with an adhesive.

[Driving and Evaluation of Display Characteristics]

A colored (black) display and a non-colored (white) display were changedover by selecting a desired set of the stripe form electrodes, passing acurrent in an amount of 2 μC per one pixel to the display electrodes for0.1 sec, to cause a reduction reaction on the display electrode side, atthe time of color formation, and passing the same amount of current tocause oxidation at the time of decoloring. The reflectance at the timeof non-coloring (white) was 85%, and the optical density (OD) of thedisplay portion at the time of color forming (black) was about 1.4(reflectance: 4%). The pixels which were not selected did not undergocolor forming or decoloring.

[Current-Voltage Characteristic]

The display apparatus having the above-mentioned structure using silverions as metal ions and using ITO for the transparent electrodes wassubjected to measurement of current-voltage characteristic with thefirst electrodes set positive, to obtain the results as shown in FIG. 9.Specifically, in the case where a negative voltage is applied to thefirst electrode, the current is extremely small and negligible when thevoltage is not more than a certain level (threshold: 0.8 volt in thiscase). On the other hand, when the voltage exceeds the threshold, acurrent begins to flow, and coloration occurs at the first electrode.The current flowing condition is maintained even if the voltage isreduced to or below the threshold. Further, when a reverse voltage,i.e., a positive voltage is applied to the first electrode to cause acurrent to flow, the color gradually disappears, the current alsodecreases, and, when the color disappears completely, the currentbecomes extremely small. The presence of such a hysteresis in thecurrent-voltage characteristic is a feature of this device.

When voltage scanning while limiting the negative voltage range isapplied to the current-characteristic shown in FIG. 9, the resistance inthe vicinity of zero voltage is higher than that in the case of completecoloration by scanning at a sufficient voltage, as shown in FIG. 10. Theresults of monitoring the resistance (complex impedance) by actuallyapplying an AC voltage of 0.1 V are shown in FIG. 11. It is seen thatthe complex impedance is lowered as the coloration density (opticaldensity) is increased.

[Coloration Control]

Coloration control is conducted by utilizing the above-mentionedphenomenon. First, an AC or pulse voltage at a voltage (0.1 to 0.5 V)less than the coloration threshold is applied, the resulting current,i.e., the resistance is measured (detection of condition), and then anappropriate amount of negative writing voltage is applied, wherebydensity can be controlled.

A specific example of this is shown in FIG. 12. FIG. 12 shows the pulsevoltage impressed on the pixel, the current flowing due to theapplication of the pulse voltage, and the optical density (OD) of thepixel. In the current value, the line indicated by broken line is astandard current value serving as a desired optical density level. Thestandard current value is preset based on FIG. 11 above.

In this embodiment, first, prior to writing, a detection voltage pulsev₁ of 0.1 volt and 10 ms is impressed, and the current i₁ (resistance)is measured. Here, since the current i₁ is less than the above-mentionedstandard current, a negative writing voltage pulse P₁ is impressed. Thenegative writing voltage pulse P₁ is −1 V and 10 ms here. When thenegative writing voltage pulse P₁ is impressed, a current I_(w) flows,to cause deposition of silver. After the application of the writingvoltage pulse P₁, a detection voltage pulse v₂ of 0.1 volt and 10 ms isagain impressed, and the current i₂ (resistance) is measured. Althoughthe density is raised by one step, the density is insufficient ascompared with the preset density, and the current i₂ is also below thestandard current value; therefore, a writing voltage pulse P₂ is againimpressed. This cycle is repeated until the current in upon applicationof the detection voltage pulse v_(n) exceeds the standard current value.When the current i_(n) upon application of the detection voltage pulsev_(n) has exceeded the standard current value, the application of thewriting voltage pulse is stopped at that time. This makes it possible tocolor the pixel in a predetermined density.

[Decoloration Control]

In the case of decoloring a colored pixel, a positive voltage isimpressed to cause an appropriate current to flow, whereby silver isdissolved and, as a result, the pixel becomes transparent. However, ifthe positive voltage is impressed excessively, not only decolorationoccurs but also iodine is librated to cause coloration in yellow, in thecase where silver iodide is used as a source of dissolved ions, forexample. In order to achieve appropriate decoloration while preventingthis phenomenon, the detection voltage pulse is impressed, the resultingcurrent is detected, and decoloration is controlled, in the same manneras in the above-mentioned coloration control.

A specific example of this is shown in FIG. 13. FIG. 13 shows the pulsevoltage impressed on the pixel, the current flowing due to theapplication of the pulse voltage, and the optical density (OD) of thepixel. In the current value, the line indicated by dot-dash line is astandard current value serving as a desired decoloration density level.This standard current value is also preset based on FIG. 11 above, butthis is less than the standard current at the above-mentioned coloration(optical) density level.

When the detection voltage pulse v₁ of 0.1 volt and 10 ms is impressedon the colored pixel and the current i₁ (resistance) is measured, thecurrent i₁ is in excess of the above-mentioned standard current value,so that it is detected that the pixel is in the colored state. In viewof this, a positive erasing voltage pulse P₁ for erasure is impressed.The positive erasing voltage pulse P₁ is 1 V and 10 ms here. When thepositive erasing voltage pulse P₁ is impressed, an erasing current I_(E)flows, and silver is dissolved. After the application of the erasingvoltage pulse P₁, a detection voltage pulse v₂ of 0.1 volt and 10 ms isagain impressed, and the current i₂ (resistance) is measured. Althoughthe density is lowered by one step, the density is insufficient ascompared with the preset erased state, and the current i₂ also is inexcess of the above-mentioned standard current value. In view of this,an erasing voltage pulse P₂ is again impressed. This cycle is repeateduntil the current i_(n) upon application of the detection voltage pulsev_(n) becomes less than the standard current value. When the currenti_(n) upon application of the detection voltage pulse v_(n) is decreasedto or below the standard current value, the application of the erasingvoltage pulse is stopped. This makes it possible to sufficiently lowerthe pixel density to a predetermined density and to securely obtain anerased state.

[Fading Control]

In the display apparatus as above, by returning the voltage to zero oropening the circuit after coloration, the colored state can bemaintained for a certain length of time (several tens of minutes at roomtemperature). However, silver is dissolved and the optical density isthereby lowered, with the lapse of time. In view of this, appropriatewriting according to the deficient density is conducted, whereby thedensity is maintained for a long time.

A specific example of this is shown in FIG. 14. FIG. 14 shows the pulsevoltage impressed on the pixel, the current flowing due to theapplication of the pulse voltage, and the optical density (OD) of thepixel. In the current value, the line indicated by dot-dash line is astandard current value serving as a desired optical density level, andis preset based on FIG. 11 above.

The writing is the same as in the above case of coloration control, andthe application of the writing voltage pulse P_(n) is repeated until thecurrent i_(n) upon application of the detection voltage pulse v_(n)exceeds the standard current value. After the writing, also, thedetection voltage pulse v_(n) is impressed at a predetermined interval,and the optical density level is monitored through the current i_(n). Inthis period, silver is dissolved with the lapse of time, the opticaldensity level is gradually lowered, and the current i_(n) is alsogradually lowered. An additional writing pulse P_(t) is impressed whenthe current i_(d) upon application of the detection voltage pulse v_(d)is decreased to or below the standard current value, as shown in thefigure. When monitoring is conducted by periodically impressing thestate detecting pulse after writing and the additional writing pulse isimpressed as required, in this manner, the optical density level can bemaintained for a long time.

[Rewriting Control]

In rewriting a black-white two-valued screen, rewriting to the nextscreen is conducted according to the condition of the current screen bythe following technique. An example of monitoring the black/whitecondition before writing and controlling the rewriting of the screen isshown in FIG. 15.

First, a state recognition pulse PS1 is impressed on each pixel, andbased on the resulting current, it is discriminated whether the displaystate of the pixel is white or black (S1). An AC or pulse voltage at avoltage (0.1 to 0.5 V) less than the coloration threshold is impressedas the state recognition pulse PS1, and the resulting current, i.e., theresistance is measured, whereby the colored state can be discriminated.For example, a smaller current (higher resistance) corresponds to white,and a larger current (lower resistance) corresponds to black.

As a result of the discrimination, in the pixel in the white displaystate W, when the next screen is also “white”, the white display stateis maintained by a white display maintaining command WH1. Incidentally,in this instance, white state discrimination WS1 is conducted in thesame technique as in the above erasure control, and when the pixeldensity is higher, an erasing operation is conducted by an erasing pulseinput ER1. On the other hand, in the pixel in the white display state W,when the next screen is “black”, writing is conducted by a black displaycommand BH1. At the time of writing, black state discrimination BS1 isconducted in the same technique as in the above coloration control, andwhen the optical density is deficient, the writing pulse input WR1 isrepeated until a predetermined optical density is obtained.

As a result of the above discrimination, in the pixel in the blackdisplay state B, when the next screen is also “black”, the black displaystate is maintained by a black display maintaining command BH2. In thisinstance, black state discrimination BS2 is conducted in the sametechnique as in the above coloration control, and when the opticaldensity is deficient, writing pulse input WR2 is repeated until apredetermined optical density is obtained. Where the next screen is“white”, white display, or erasure, is conducted by a white displaycommand WH2. In this instance, white state discrimination WS2 isconducted by the same technique as in the above erasure control, andwhen an unerased portion is left, an erasing operation is conducted byuse of an erasing pulse input ER2 until a predetermined white level isobtained.

[Gradation Control]

Gradation control can be realized by applying the above-mentionedcoloration control. An example of monitoring the density before writingand then controlling to a predetermined density is shown in FIG. 16.

In the case of performing gradation control, first, a state recognitionpulse PS is impressed on the pixel, and the initial density state isgrasped (SN). The initial density state can be discriminated byimpressing an AC or pulse voltage at a voltage (0.1 to 0.5 V) less thanthe coloration threshold and measuring the resulting current, orresistance, in the same manner as in the above case of colorationcontrol or erasure control.

Next, the density of each pixel is brought to a predetermined gradationdensity by an arbitrary density display command N. In this case, itsuffices to set each gradation density and the relevant resistance value(current value) in correspondence with each other, and to performdensity control based on this. The density control is conducted whilemonitoring the colored state of the pixels by state discrimination JS,in the same manner as in the case of coloration control or erasurecontrol. The state discrimination JS is conducted based on the currentvalue according to each gradation density, and the individual pixels areset to predetermined gradation densities. Based on the results of thestate discrimination JS, the density is lowered by erasing pulse inputER when the density is high, and the density is raised by writing pulseinput WR when the density is low.

As is clear from the above description, according to the presentinvention, it is possible to quantitatively control the optical density(coloration density) and to control the colored state of each pixel toan appropriate state, for example, a uniform state. Besides, by thequantitative control, display of gradations can also be realized.

Further, according to the present invention, the optical density can bequantitatively controlled even in the erasing operation, so that theerasing and writing times can be shortened and the power consumption canbe minimized, as compared with a method in which coloration is conductedafter complete erasure. Particularly in the case of black-whitetwo-valued display or gradation display based on gray scale, rewritingof the screen can be performed with minimum power consumption.Furthermore, according to the present invention, the phenomenon in whichthe optical density is lowered with the lapse of time can be accuratelycompensated for, and the attendant power consumption can be minimized.

1. A method of driving a display apparatus for displaying an imagethrough deposition and dissolution of a metal by impressing apredetermined voltage of each pixel, wherein a voltage pulse oralternating current voltage having an amplitude of not more than athreshold for deposition of said metal is impressed, and, according tothe resulting current, a writing pulse or an erasing pulse iscontrolled.
 2. The method of driving a display apparatus as set forth inclaim 1, wherein by controlling said writing pulse or erasing pulse, thedensity of each said pixel is controlled, thereby performing display ofgradations.
 3. The method of driving a display apparatus as set forth inclaim 1, wherein said display apparatus has a structure in which opposedelectrodes are disposed respectively on both sides of an electrolytelayer containing metal ions, a current is passed between said opposedelectrodes to thereby deposit the metal on one of said electrodes as acoloring material, and said pixel where said metal is deposited isrecognized as a colored state.
 4. The method of driving a displayapparatus as set forth in claim 3, wherein said coloring material is atleast one selected from the group consisting of bismuth, copper, silver,lithium, iron, chromium, nickel, and cadmium.
 5. The method of driving adisplay apparatus as set forth in claim 3, wherein said electrolytelayer is a polymer electrolyte layer.
 6. The method of driving a displayapparatus as set forth in claim 3, wherein said electrode on which saidmetal is deposited is a transparent electrode.
 7. A method of driving adisplay apparatus for displaying an image through deposition anddissolution of a metal by impressing a predetermined voltage on eachpixel, wherein after writing is conducted by impressing a voltage of notless than a threshold for deposition of said metal, a voltage pulse oralternating current voltage having an amplitude of not more than athreshold for deposition of said metal is impressed, and, when theresulting current is decreased to or below a predetermined value, anadditional writing pulse is impressed.
 8. A method of driving a displayapparatus for displaying an image through deposition and dissolution ofa metal by impressing a predetermined voltage on each pixel, wherein avoltage pulse or alternating current voltage having an amplitude of notmore than a threshold for deposition of said metal is impressed, thewriting state of each said pixel is detected according to the resultingcurrent, and a rewriting operation is selected according to thedetection results.